Canonicalize -1 - x to ~x.
Instcombine does this but apparently there are situations where this pattern will escape the optimizer and / or created by isel. Here is a case that's seen in JavaScriptCore:
%t1 = sub i32 0, %a
%t2 = add i32 %t1, -1
The dag combiner pattern: ((c1-A)+c2) -> (c1+c2)-A
will fold it to -1 - %a.
git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@93773 91177308-0d34-0410-b5e6-96231b3b80d8
diff --git a/lib/CodeGen/SelectionDAG/DAGCombiner.cpp b/lib/CodeGen/SelectionDAG/DAGCombiner.cpp
index 549527c..7fe908c 100644
--- a/lib/CodeGen/SelectionDAG/DAGCombiner.cpp
+++ b/lib/CodeGen/SelectionDAG/DAGCombiner.cpp
@@ -1176,6 +1176,9 @@
if (N1C)
return DAG.getNode(ISD::ADD, N->getDebugLoc(), VT, N0,
DAG.getConstant(-N1C->getAPIntValue(), VT));
+ // Canonicalize (sub -1, x) -> ~x, i.e. (xor x, -1)
+ if (N0C && N0C->isAllOnesValue())
+ return DAG.getNode(ISD::XOR, N->getDebugLoc(), VT, N1, N0);
// fold (A+B)-A -> B
if (N0.getOpcode() == ISD::ADD && N0.getOperand(0) == N1)
return N0.getOperand(1);